[#318] Fix panic on invalid multipart form

Previously, simple 'curl -X POST http://localhost:8084/test' leads to panic because of wrong handle matching

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/handler/put.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"encoding/xml"
+	stderrors "errors"
 	"fmt"
 	"io"
 	"net"
@@ -628,11 +629,21 @@ func checkPostPolicy(r *http.Request, reqInfo *middleware.ReqInfo, metadata map[
 		policy.empty = false
 	}
 
+	if r.MultipartForm == nil {
+		return nil, stderrors.New("empty multipart form")
+	}
+
 	for key, v := range r.MultipartForm.Value {
-		value := v[0]
 		if key == "file" || key == "policy" || key == "x-amz-signature" || strings.HasPrefix(key, "x-ignore-") {
 			continue
 		}
+
+		if len(v) != 1 {
+			return nil, fmt.Errorf("empty multipart value for key '%s'", key)
+		}
+
+		value := v[0]
+
 		if err := policy.CheckField(key, value); err != nil {
 			return nil, fmt.Errorf("'%s' form field doesn't match the policy: %w", key, err)
 		}